Silly tomato question...

[Fucksandflowers]

I know they are technically tender perennial...

Is Milton Keynes far enough south to risk overwintering outside if mulched or are they guaranteed to die on me?

they will die. usually mid september they don't get enough light.
in a greenhouse with a grow light it might be possible to keep it growing.

They’ll die, they really are very tender. Either that or they’ll get blight and you won’t be able to plant any tomatoes/peppers/chillis/potatoes for several years. Much easier to treat them as an annual, they’re very easy to grow from seed.

Aw, What a shame!

They are treated as an annual in the Mediterranean - they crop so heavily from seed in one year I really wouldn't worry!